Satellite observations of mean winds and tides in the lower thermosphere: 2. Wind Imaging Interferometer monthly winds for 1992 and 1993

Satellite observations of monthly (i. e., 60-d averages centered on the 15th of each month) zonal mean and migrating tide winds in the region of 90-120 km and 40 degrees S -40 degrees N from the Wind Imaging Interferometer are presented. The zonal mean zonal winds are characterized by annually varying eastward winds at midlatitudes with a maximum in summer and semiannually varying westward winds in the tropics with maxima at equinox below 105 km. The zonal mean meridional winds are characterized by a summer-to-winter flow below 100 km at solstice and by a cell-like structure at equinox; the latter is the first such global observation of this tidally driven structure. Two distinct types of diurnal tide are observed: an upward propagating one and an evanescent one of comparable magnitude. The propagating tide, which is dominant in the subtropics below about 105 km, exhibits a semiannual variation with a primary maximum in March/April and a secondary maximum in September/October. The evanescent tide, which is dominant poleward of 20 degrees N/ S and above 100 km, has an annual variation with a maximum in the summer months. The coupling of the two produces a double-peaked structure in the vertical profile of the diurnal amplitude. The semidiurnal tide is generally weaker than the diurnal tide. The maximum semidiurnal winds are at 30- 40 degrees N/ S and at 105-110 km. Both zonal and meridional components are stronger in April-September than in October March in both hemispheres, but in the equatorial region in April-September the meridional component has a maximum whereas the zonal component has a minimum.